The invention relates to an operating device for gas discharge lamps as claimed in the precharacterizing clause of claim 1. This relates in particular to an improvement to the half-bridge inverter contained in the operating device, and to its drive. The invention furthermore relates to simplification of a switching-off device for the operating device, and to low-cost power factor correction for the current drawn from the mains.
The document EP 0 093 469 (De Bijl) describes an operating device for gas discharge lamps, which represents the prior art. This operating device contains a free-running half-bridge inverter, which uses a DC voltage to produce a high-frequency AC voltage by switching an upper and a lower half-bridge transistor, which are connected in series, on and off alternatively. The DC voltage is generally produced by means of a bridge rectifier, comprising four rectifier diodes, from the mains voltage. In this context, free-running means that the drive for the half-bridge transistors is obtained from a load circuit, and that no independently oscillating oscillator circuit is provided to produce said drive. Said drive is preferably obtained by means of a current transformer. A primary winding of the current transformer is arranged in the load circuit and a load current flows through it which is essentially equivalent to the load current, which can essentially be equated to the current which is emitted from the half-bridge inverter. One secondary winding of the current transformer is arranged in each of two drive circuits, which each produce a signal which is supplied to the control electrodes of the half-bridge transistors. The load circuit is connected to the connection point of the half-bridge transistor. The main component of the load circuit is a lamp inductor, to which gas discharge lamps can be connected in series, via terminal connections. It is also possible to connect a number of load circuits in parallel; the primary winding can then be arranged such that the total current from all the load circuits flows through it.
Each of the drive circuits produces a feedback signal, which is essentially proportional to the load current. Ideally, the secondary windings must be short-circuited for this purpose, but in practice they are terminated with a low impedance. Otherwise, either saturation phenomena would occur in the current transistor or the primary winding would have an undesirably strong influence on the load circuit. According to the prior art, bipolar transistors are used for the half-bridge transistors, drawing their drive from the secondary windings. The base connection of the bipolar transistors, which is used as a control electrode, naturally has a sufficiently low impedance to avoid the abovementioned effects.
The voltage drop across the secondary windings in the abovementioned conditions represents a measure of the load current and, in the prior art, forms feedback signals. These are in each case supplied to a timer which, in the simplest case, comprises a timing capacitor and a timing resistor connected in series. If the respective timing capacitor is charged to an integration value which is sufficient to drive a switching-off transistor, the respective half-bridge transistor is switched off.
A resonance capacitor, which together with the lamp inductor forms a resonance circuit, is effectively connected in parallel with a gas discharge lamp and in series with the lamp inductor, in particular in order to start gas discharge lamps. This resonance circuit is operated close to its resonance point for starting, thus resulting in a voltage which is sufficiently high to start a gas discharge lamp being formed across the resonant capacitor.
A high current is accordingly formed in the lamp inductor and thus in the half-bridge transistors. In order to avoid components being overloaded, the amplitude of the load current is limited in the prior art. This is done via in each case one first voltage threshold value switch, which is connected in parallel with the respective timing resistor. If the load current rises above a predetermined level, then the respective feedback signal reaches a value which can break through the respective first voltage threshold value switch, thus leading to the respective half-bridge transistor being switched off immediately.
The object of the present invention is to provide an operating device for gas discharge lamps as claimed in the precharacterizing clause of claim 1, which makes the topology described in the prior art feasible not only for half bridges with bipolar transistors, which require a drive current of course, but also allows voltage controlled semiconductor switches such as MOF field-effect transistors (MOSFET) to be used. The object on which this problem is based essentially includes the provision of a drive signal for the semiconductor switches which is proportional to the load current.
This object is achieved by an operating device for gas discharge lamps having the features of the precharacterizing clause of claim 1 and by means of the features of the characterizing part of claim 1. Particularly advantageous refinements can be found in the dependent claims.
Bipolar transistors are increasingly being replaced by voltage controlled semiconductor switches such as MOSFETs and IGBTs, mainly for cost reasons.
If one of the secondary windings described above is used to drive a voltage controlled semiconductor switch rather than a bipolar transistor, then the termination of the secondary winding no longer has a low impedance but a high impedance, and the disadvantages mentioned in the section relating to the prior art occur. According to the invention, the drive circuits are each equipped with a second voltage threshold value switch, which has a second voltage threshold and is connected in parallel with the secondary winding. In the simplest case, the second voltage threshold value switch comprises a zener diode and a current measurement resistor connected in series, with the zener diode having a zener voltage which corresponds to the second voltage threshold. If the voltage across the secondary winding rises, starting from zero, then the second voltage threshold value switch initially has no effect. On reaching the second voltage threshold, the zener diode starts to conduct, and the secondary winding is terminated with a low impedance, as desired. The value of the second voltage threshold must be lower than a threshold voltage which the voltage controlled semiconductor switch requires, as a minimum, as a drive. The size of the current measurement resistor has to satisfy two conditions. Firstly, the value of the current measurement resistor must be small enough to ensure a low-impedance termination on the secondary winding. Secondly, the value of the current measurement resistor must be high enough to allow the voltage across the secondary winding to rise further as far as the first voltage threshold.
Since a current which is essentially proportional to the load current flows in the current measurement resistor according to the invention, the voltage across the current measurement resistor is, of course, also a measure of the load current. The voltage across the current measurement resistor may thus be used, according to the invention, in order to detect a fault situation. For this purpose, it is supplied to a switching-off device. In order to suppress interference, the time average of the voltage across the current measurement resistor is formed in the switching-off device. If this exceeds a given limit value, the switching-off device prevents further oscillation of the half-bridge inverter. This is done in particular by suppressing the drive signal for one of the two half-bridge transistors.
The operating devices under discussion generally have two mains voltage terminals which can be connected to a mains voltage, thus allowing a mains current to flow. Relevant standards (for example: IEC 1000-3-2) specify maximum amplitudes for the harmonics in the mains current. In order to comply with these Standards, operating devices have so-called PFC circuits (Power Factor Correction). One low-cost implementation for these PFC circuits is represented by so-called pumping circuits, as are described, for example, in EP 253 224 (Zuchtriegel) or EP 1 028 606 (Rudolph). If a pumping circuit is combined with a free-running half-bridge inverter according to the prior art, this leads to problems in producing the necessary starting voltage for the gas discharge lamps, and problems due to the high power losses during switching of the half-bridge transistors. Said problems occur in particular in the case of high-power gas discharge lamps. One reason for this, inter alia, is the storage times, which are typical for bipolar transistors and do not allow the switching-off time to be defined exactly. The present invention allows the use of voltage controlled semiconductor switches such as MOSFETs, which have no storage times and therefore allow said problems to be avoided. This means that the half-bridge inverter according to the invention in conjunction with a pumping circuit can be used advantageously even for a load which consumes a power of more than 100 W.
A further effect which occurs in the case of the half-bridge inverter according to the invention with a pumping circuit is the heavy modulation of the operating frequency by the mains voltage, which is subject to the oscillation of the half-bridge inverter. Depending on the instantaneous value of the mains voltage, said operating frequency is within a frequency band which has a bandwidth of more than 10 kHz. The electromagnetic interference caused by an operating device according to the invention is thus distributed over a wide frequency band. The amount of energy reaching an appliance that is subject to interference is thus advantageously low. Furthermore, the complexity for suppression of an operating device according to the invention can be kept low.
A further advantageous application of the current measurement resistor according to the invention is in the starting circuit for the free-running half-bridge inverter. In order to start the half-bridge inverter, the normal process is to charge a starting capacitor and, when a trigger voltage is reached across the charge-storage capacitor, to discharge a portion of the charge stored in the charge-storage capacitor via a trigger element to the control electrode of a half-bridge capacitor. In this case, one problem that can occur is that the charge pulse produced in this way at the relevant control electrode is too short and too small, and continued oscillation of the half-bridge inverter is not triggered. According to the invention, a portion of the stored charge in the charging capacitor is supplied via a diode to the current measurement resistor according to the invention. This makes it possible to ensure that the half-bridge inverter starts to oscillate reliably.